From the past, there are surveillance cameras that include a day mode in which images are captured under natural daylight or under lightings of visible light such as white light and a night mode in which images are captured by irradiating infrared light.
In the day mode, color images can be captured. On the other hand, in the night mode, reflected light of irradiated infrared light, that has been reflected by a subject, is received, so color information of R, B, G, and the like cannot be obtained, and only a light reception intensity of the reflected light (infrared light) can be obtained. Therefore, the obtained image becomes a monochrome image including gray, green, and the like.
Incidentally, when considering usages of surveillance cameras, it is desirable for color images to be obtained even in the night mode.
In this regard, various methods for generating a color image on the basis of an image obtained by capturing a state where infrared light is irradiated onto an image pickup range (subject) (hereinafter, referred to as infrared image) have been proposed from the past.
For example, Patent Literature 1 discloses a method of generating a color image on the basis of an infrared image obtained by irradiating 3 types of infrared light having different wavelengths. This method uses the fact that a reflectance in a case where infrared light having wavelengths of 780 nm, 870 nm, and 940 nm are irradiated onto a resin and a reflectance in a case where visible light of R, G, and B are irradiated have a positive correlation. By this method, as long as the subject is a resin, a color image having high color reproducibility can be obtained.
However, in a case where the subject is other than a resin, while reproducibility of R components of a color image to be generated is relatively high, G and B components cannot be reproduced in their original colors, so this method is inapplicable to arbitrary subjects other than a resin.
Further, for example, Patent Literature 2 discloses a method of capturing an image by irradiating infrared light in a state where a subject is barely visible under a low-illuminance lighting or the like. In this method, an image in which visible light and infrared light are mixed is captured, and color information is reproduced as parameters of white balance processing applied to this image are changed from values applied to a normal image that uses only visible light.
In this method, however, there is an essential problem that infrared light and visible light cannot be separated from each other, so accuracy of color information to be reproduced is deteriorated due to an influence of infrared light.
Furthermore, for example, Patent Literature 3 and Non-Patent Literature 1 disclose technologies for consecutively capturing an infrared image in a state where infrared light is irradiated and a visible image under low illuminance where infrared light is not irradiated and generating a color image using those images.